2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a widespread environmental contaminant, is associated with delays in prubertal development in animal studies. On 10 July 1976, as a result of a chemical explosion, residents of Seveso, Italy, experienced the highest levels of TCDD exposure experienced by a human population. Twenty years later, we initiated the Seveso Women's Health Study (SWHS), a retropestive cohort study of female residents of the most contaminated areas, to determine wether the women were at higher risk for reproductive disease. We examined the association of TCDD serum levels, based on measurements in serum levels, based on measurements in serum collected soon after the explosion, with reported age at menarche among the 282 SWHS women who were premenarheal at the time of explosion. We found no change in risk of onset of menarche with a 10-fold increase in TCDD (e.g., 10-100 ppt; hazard ratio = 0.95; 95% confidence interval, 0.83-1.09; p-value for trend = (p-0.65). In summary, we found that individual serum TCDD measurements are not significantly related to age at menrche among women in the SWHS cohort. The women in this study experienced substantial TCDD exposure during postnatal but prepubertal developmental period. Given that animal evidence suggests in utero exposure has the most significant effect on onset of puberty, continued follow-up of the offspring of the SWHS cohort is important.